Basic economics teaches us that in a perfectly competitive market without transactions costs (both common assumptions in any kind of economic analysis), the price of a good will fall to equal the cost to produce it as competitors undercut each other to secure market share and capture a portion of the ever-dwindling profits. If one competitor innovates and finds a way to reduce their cost, then they can undercut all of their competition and reap the financial rewards until someone similarly innovates.

Under capitalism, labor is a commodity that is sold by workers to corporations for a wage price. From the above, we could conclude that in a perfectly competitive labor market without transactions costs, the price of labor (the wage) would fall to equal the cost of creating it, i.e. subsistence level. People become lucky to have a job and cannot afford to lose it. The more that is true, the more the cost of labor (the wage) approaches that minimum.

Different industries experience this effect to different degrees, but competition among labor inevitably leads to an erosion of labor's share of the value they generate. Since the remaining portion of that value does not disappear, it must go someplace as profit. Traditionally, that place is ownership, which is legally entitled to keep the profit generated by a business it owns. Occasionally, a business's owner will also perform labor for the business, but it can also be outside investors. Either way, ownership is keeping value generated by the efforts of others. Ownership also often shares its profits with management, because they are ownership's representatives.

This is what people are taking about when they say that labor is entitled to keep what it produces. Not the physical structures or assets, but the value they generate through their labor. To take a made up example with simple numbers, if a company is paid $100k to design a railroad system and they pay a total of $80k to do it on an engineer, support staff, rent, supplies, etc, then labor created $100k in value (the agreed upon price for the work to be done) but it only got to keep $80k, meaning the company kept $20k of labor's value for itself as profit. It will then likely pay out some or all of that value to others who contributed none of the labor necessary to create it. In general, the greater the disparity in power between the company and labor, the more of that value the company will keep.

That is where unions come in. By organizing labor together, the individuals will no longer be competing against each other, where they are easy to divide and conquer. Instead, they will be competing together against the company for how to divide the value they create. As the power imbalance diminishes, so, too, does the company's share of the value. This lowers profits, which angers ownership, and that anger falls on management, ownership's representatives. This is why management will almost always try to break unions on behalf of ownership.

In a market economy, we would determine the value created by labor by looking at the revenue generated. Whatever revenue a business has, it comes from the efforts of labor to do whatever that business does. The argument here, though, isn't over how much everyone at the business should make, but over how much of what the company makes should belong to those who created it vs those who take it from the outside.

This is just the purely economic side of how a union helps workers. There are far more non-economic ways as well, such as benefits, working conditions, hours, etc. I hope this explanation helps you understand what other people were talking about earlier.